This invention relates to an electrochemical method for the bromination of hydroxy aromatic compounds. More particularly the present invention provides a method for the preparation of brominated phenols such as para-bromophenol.
Brominated hydroxy aromatic compounds such as para-bromophenol are valuable intermediates for production of bisphenols such as 4,4xe2x80x2-biphenol and hydroquinone. 4,4xe2x80x2-biphenol may be prepared by coupling para-bromophenol, and hydroquinone may be prepared by hydrolysis of para-bromophenol. In addition, brominated phenols are versatile intermediates in the preparation of organic dyestuffs, and as synthons for agricultural chemicals used for plant protection. It is known that molecular bromine, Br2, reacts readily with phenol to form predominantly bromophenol. However, although the reaction rate is high, the selectivity of the reaction is relatively poor, para/ortho selectivity of only about 75% being typical. In addition, significant amounts of the xe2x80x9cover-brominatedxe2x80x9d product, 2,4-dibromophenol, are formed. It should be noted as well that the use and handling of molecular bromine, a volatile, toxic liquid at room temperature, poses significant engineering challenges to prevent its adventitious release into the environment, as well as human health concerns related to the acute toxicity of molecular bromine.
Various attempts have been made to improve selectivity in the bromination of phenol with molecular bromine, and in some aspects these efforts have been successful. Thus, the use of tetraalkylammonium salts of the Br3xe2x88x92anion afforded improved selectivity during the bromination of phenol. However, the improved selectivity came at the expense of low reaction rates. Moreover, the tetraalkylammonium bromides are costly and must be recovered and recycled. Other salts of the tribromide anion (Br3xe2x88x92) have also been used for bromination reactions of phenol. Complexes of molecular bromine with alkylsulfides have demonstrated good selectivity for para-bromination of phenol but possess many of the same disadvantages as the organic salts of tribromide anion. Other schemes to improve selectivity in bromination reactions of phenol have employed combinations of molecular bromine with silica, and molecular bromine and cyclodextrin. Here again, however, selectivity was insufficient. Moreover, bromination reactions based upon molecular bromine or its complexes use only half of the bromine introduced and produce a full equivalent of hydrogen bromide, HBr, as a by-product. To date, the highest selectivity observed in the bromination of phenol was achieved by reaction of phenol with a brominating agent based upon a combination of HBr, molecular oxygen, and a heteropolyacid catalyst. Despite the high selectivity observed (99%) in the bromination of phenol, the process suffers from low catalyst turnover, the high molecular weight of the catalyst, and the high cost of the catalyst.
Some years ago, it was reported that the electrolysis of aqueous solutions of phenol in the presence of a bromide salt as the electrolyte afforded predominantly para-bromophenol (T.Bejerano, E.Gileadi, Electrochimica Acta, 1976, vol. 21, p. 231). The method disclosed only very low concentrations of reactants suggesting that such a method was unlikely to be useful for the preparation of substantial amounts of bromophenol product. Moreover, the low para/ortho selectivity observed cast doubt upon the method""s viability in as a modern industrial practice.
It is clear from the foregoing discussion that new processes which are not dependent upon the use of molecular bromine for the preparation of brominated hydroxy aromatic compounds such as bromophenol represent very attractive goals, especially if the new processes are both highly selective and efficient. The present invention provides a new, highly selective and highly efficient electrochemical method for the preparation of brominated hydroxy aromatic compounds. The new method does not require the use of molecular bromine.
In one aspect the present invention relates to a method for the preparation of brominated hydroxy aromatic compounds, said method comprising: electrolyzing in an electrochemical cell a mixture comprising a hydroxy aromatic compound, at least one source of bromide ion, at least one organic solvent, and optionally water, to provide a product brominated hydroxy aromatic compound.
In another aspect the present invention provides an electrochemical method for the preparation of bromophenols such as para-bromophenol and 4-bromo-2-methylphenol. In yet another aspect the method of the present invention comprises recovering the product brominated hydroxy aromatic compound from a product mixture.